On Wed, Jul 30, 2025 at 8:24 PM Andrew Pinski <pins...@gmail.com> wrote:
>
> On Wed, Jul 30, 2025 at 5:33 PM H.J. Lu <hjl.to...@gmail.com> wrote:
> >
> > According to the TLS spec, linker can optimize the TLS access by
> > overriding the TLS access model. Do the same in compiler by using
> > the default TLS mode if it is more optimized than the TLS model
> > attribute.
>
> So reading the old bug reports specifically the comment here:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78294#c18 .
> This is by design of the attribute. And should not be changed. And the
> problem is mismatch of the decls with and without the tls_model
> attribute and merging them. Jakub specifically brings up should we
> warn if they are mismatched? RIght now your patch does causes what the
> user asked for to be overridden. And there is no way to force the
> model and such.
The tls_model attribute is designed specifically for the ELF TLS spec which
specifies TLS model access optimization. When
__attribute__ ((tls_model ("initial-exec")))
is used, it specifies the least optimized TLS access model, not THE
TLS access model. Linker is free to optimize. So should GCC.
> Also from the source of the glibc issue, it is a linker issue in a
> similar way of the gold issue was with
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78294 .
>
> Thanks,
> Andrew Pinski
>
> >
> > gcc/
> >
> > PR c/121318
> > * doc/extend.texi: Update the tls_model attribute.
> >
> > gcc/c-family/
> >
> > PR c/121318
> > * c-attribs.cc (handle_tls_model_attribute): Use the default TLS
> > mode if it is more optimized.
> >
> > gcc/testsuite/
> >
> > PR c/121318
> > * gcc.dg/tls/pr121318.c: New test.
> >
> > Signed-off-by: H.J. Lu <hjl.to...@gmail.com>
> > ---
> > gcc/c-family/c-attribs.cc | 4 ++++
> > gcc/doc/extend.texi | 8 +++++++-
> > gcc/testsuite/gcc.dg/tls/pr121318.c | 16 ++++++++++++++++
> > 3 files changed, 27 insertions(+), 1 deletion(-)
> > create mode 100644 gcc/testsuite/gcc.dg/tls/pr121318.c
> >
> > diff --git a/gcc/c-family/c-attribs.cc b/gcc/c-family/c-attribs.cc
> > index 1f4a0df1205..d49d00c7471 100644
> > --- a/gcc/c-family/c-attribs.cc
> > +++ b/gcc/c-family/c-attribs.cc
> > @@ -3664,6 +3664,10 @@ handle_tls_model_attribute (tree *node, tree name,
> > tree args,
> > name,
> > "local-exec", "initial-exec", "local-dynamic", "global-dynamic");
> >
> > + /* Use the default TLS mode if it is more optimized. */
> > + tls_model default_kind = decl_default_tls_model (decl);
> > + if (default_kind > kind)
> > + kind = default_kind;
> > set_decl_tls_model (decl, kind);
> > return NULL_TREE;
> > }
> > diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> > index 224d6197d63..b204d1ce02c 100644
> > --- a/gcc/doc/extend.texi
> > +++ b/gcc/doc/extend.texi
> > @@ -7528,7 +7528,13 @@ The @code{tls_model} attribute sets thread-local
> > storage model
> > overriding @option{-ftls-model=} command-line switch on a per-variable
> > basis.
> > The @var{tls_model} argument should be one of @code{global-dynamic},
> > -@code{local-dynamic}, @code{initial-exec} or @code{local-exec}.
> > +@code{local-dynamic}, @code{initial-exec} or @code{local-exec}. The
> > +@code{tls_model} attribute specifies the least optimized @acronym{TLS}
> > +access model. GCC may choose a more optimized @acronym{TLS} access
> > +model, according to
> > +@uref{https://www.akkadia.org/drepper/tls.pdf,
> > +ELF Handling For Thread-Local Storage}, ignoring the @code{tls_model}
> > +attribute.
> >
> > Not all targets support this attribute.
> >
> > diff --git a/gcc/testsuite/gcc.dg/tls/pr121318.c
> > b/gcc/testsuite/gcc.dg/tls/pr121318.c
> > new file mode 100644
> > index 00000000000..68378635f1d
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.dg/tls/pr121318.c
> > @@ -0,0 +1,16 @@
> > +/* { dg-do compile { target { tls && pie } } } */
> > +/* { dg-options "-O2 -fPIE" } */
> > +
> > +extern const int afoo[3];
> > +
> > +__thread const int *pfoo __attribute__ ((tls_model ("initial-exec"))) =
> > afoo;
> > +
> > +const int **
> > +ppfoo (void)
> > +{
> > + return &pfoo;
> > +}
> > +
> > +/* { dg-final { scan-assembler "addq\[ \t\]\\\$pfoo@tpoff, %rax" { target
> > { { i?86-*-linux* x86_64-*-linux* } && { lp64 } } } } } */
> > +/* { dg-final { scan-assembler "leal\[ \t\]pfoo@tpoff(%rax), %eax" {
> > target { { i?86-*-linux* x86_64-*-linux* } && { x32 } } } } } */
> > +/* { dg-final { scan-assembler "leal\[ \t\]pfoo@ntpoff, %eax" { target { {
> > i?86-*-linux* x86_64-*-linux* } && { ia32 } } } } } */
> > --
> > 2.50.1
> >
--
H.J.
